Device for on-line measuring of the thickness of a continuously produced sheet

ABSTRACT

A measuring device scanning from edge to edge of a continuously fed sheet includes two sensing heads spaced apart from the sheet by air cushions whose thickness is not influenced by pressure of supplied air and is subjected to an invariable approaching gravimetric force. The air cushions are preferably formed of air jets from calibrated holes at the inside of both sensing heads, each of said holes being surrounded by annular barriers with circular rims. While the approaching force applied to the upper head is given by the own weight of the latter, the corresponding opposed force on the lower head is provided by a pressure regulator of internal air at the inside of the lower sensing head.

The present invention relates to a device for on-line measurements ofthe thickness of a continuously produced sheet. It is known that theindustries producing continuous sheet material such as paper, rubber,plastics encounter an increasing need of a measuring device able toprecisely indicate without direct contact the variations of thicknessfrom edge to edge of the sheet while it is fabricated. In fact theincreasing speed of production makes it not adequate the traditional offline tests which are normally done at the end of the roll with amicrometer on a cross machine strip.

For such a reason the world leading producers of on-line measurementsystems for sheet material have been engaged for several years in thedevelopment of thickness sensors capable of traversing continuously fromedge to edge of the sheet by meeting adequate accuracy and reliabilityrequirements. The degree of accuracy needed nowadays for thicknesssensors is in the order of a micrometer (10⁻⁶ m) or less for thethinnest sheets.

The technique generally used for this category of measuring devices isbased on two opposite sensing heads comprising a magnetic sensor ofrelative distance capable of moving on the sheet without friction. It isto be noted that, being a basic requisite of this measuring device thecontinuous scanning from edge to edge of the sheet, the sensing headsmust be mounted on two supports traversing on proper guides longer thatthe sheet width. It is worth saying that modern lines producing printingpaper can be 10 meter wide and have a speed over 1000 meter/min.

In order to make the sensing heads frictionless and to avoid theircontact with the sheet surface, the commonly adopted method is theair-cushion technique implemented by means of suitable air jets emittedfrom the surface of the sensing heads facing the sheet. The jets producea thin layer of air under pressure which maintains the heads at adistance from the sheet eliminating the friction. In this technique thedegree of accuracy of the measurements is mainly dependent on the degreeof invariability and repeatability of the thickness of the air cushion.

A device functioning according to the above principle is illustrated inpublication WO 84/02772. In this case each of the two opposite sensingheads is attached to a sliding piston moving axially with respect to theplane of the sheet, and it is urged toward the sheet by the forceexerted by the feed air pressure in a pressurized rear chamber acting onthe piston counteracting the repulsive force of a single frontal aircushion fed by the same pressure air conveyed through the hollow of thepiston via its opened far end. Since the equilibrium of the twocounteracting forces must be ensured at unchanged cushion gap, therepulsive force has to vary proportionally to the feed air pressure inthe same manner as the urging force applied to the piston.

In this described embodiment the effective air cushion is delimited bythe inner edge of the annular barrier which creates a restriction to theradial air flow causing a sharp drop of pressure between the internalarea and the ambient. The sucking phenomenon occurring in the areaimmediately outside the edge, due to Bernoulli principle, is incidentaland does not play any specific role, its sole effect being that ofreducing marginally the repulsive force exerted by the internal positivepressure area.

The own weight of the floating heads should also be considered whenevera rigorous analysis of equilibrium of forces is to be done, but it hasbeen considered negligible by this inventor because of the lightfloating head-piston assembly compared to the large counteractingpneumatic forces of several newtons.

It is an object of the present invention to provide a device of theabove-mentioned type in which the thickness of the air cushion betweensensing heads and the sheet to be measured is kept as constant aspossible independently from the pressure of air fed.

This is obtained through the features of the measuring device of thewithin invention for scanning from edge to edge of an industrial sheetcharacterized by two disks shaped hollow sensing heads which face eachother on opposite sides of the sheet. The sensing heads are pressurizedby compressed air from an external source, and are separated from thecontinuous sheet surfaces by air layers produced by multiple aircushions. The sensing heads further are mounted on pistons which slidealong a path perpendicular to the plane of the sheet plane and areguided by the bore of a pipe connected to a sealed rear chamber housingthe rear portion of each associated piston. Magnetic sensors arecentrally located in each sensing head for generating a signalproportional to the distance between the sensing heads, from which ameasurement of the thickness of the sheet can be derived. Each sensinghead also has a number of air cushions placed on its base facing thesheet symmetrically located at a suitable distance around the center toleave enough space for the central magnetic element to be out of the aircushion pressure fields. Each air cushion is formed by an outcoming jetof air from a calibrated hole located at the center of a circularrecessed land surrounded by a flange-shaped annular barrier protrudingtoward the sheet. The annular barrier has a base located next to thesheet surface and is shaped as a flat annular band which extendsoutwardly in the radial direction so that its area is substantiallylarger than the area of the inner recessed land. The annular band andthe contiguous sheet surface form an extended restricted passage for theoutwardly escaping air, causing a relevant pressure drop due to laminarfriction and a substantial concurrent compression caused by air velocitydecrease due to the radial structure of the air stream so that the aircushion pressure pattern is mainly determined by these two phenomena.The effective air cushion region is delimited by the outer periphery ofthe annular band, the two phenomena producing opposite effects tocompensate variations of fed air pressure around its nominal value,whereby no variation of the repulsive force exerted by the air cushionsat constant air cushion gaps is produced. The repulsive force acting onthe upper sensing head is counteracted by the weight of the head-pistonassembly, with the associated rear chamber open to the ambient pressure,whereas the repulsive force acting on the lower head is counteracted bythe action of the piston caused by the pressurized air in the associatedrear chamber, maintained at constant pressure by a pressure regulatorsuch that the net upward, sheet-approaching force applied to the lowersensing head equals the downward, sheet-approaching force applied to theupper sensing head.

Other objects and advantages of the device according to the inventionwill be clearer from the following description of a preferred embodimentthereof with reference to the annexed drawings in which:

FIG. 1 is a schematic sectional view of the device according to theinvention in a plane perpendicular to the sheet whose thickness is to bemeasured;

FIGS. 2 and 2a are respectively an enlarged view of a particular of FIG.1, i.e. the portion of device near to the sheet, and a graph showing theair pressure values under an annular barrier;

FIG. 3 is a plan view of a sensing head of the device according to theinvention; and

FIG. 4 is a more detailed view, similar to FIG. 1, showing only the partof device relating to the lower head.

DESCRIPTION OF PREFERRED EMBODIMENT

With reference to FIG. 1 the sensor comprises two sensing heads 1 and 2facing the opposite surface of the sheet 3, separated therefrom by twoair layers.

One of the sensing heads comprises the active member of an inductivesensor 7 generating an electric signal proportional to the distancebetween the two heads, while the other comprises the associated target(usually made of ferrite). Because the subject measuring device isdesigned to move across the sheet, it is mandatory that the heads areable to open up to a sufficient clearance to avoid a collision againstthe sheet edge when they enter the sheet. In addition, being the sheetpath is subject to flutter, i.e. making small movements in a directionperpendicular to the sheet plane mainly due to variations of sheettension, the two heads must be free to move up and down perpendicularlyto the sheet plane while measuring without changing the pressure appliedto the sheet itself. In order to obtain the above mentioned objectiveseach of the two sensing heads is mounted on a piston, respectively 5 and6, guided by the bore of an associate vertical pipe along a pathperpendicular to the sheet.

Each of said pipes is connected to a rear sealed chamber (14 and 16).The upper chamber 16 is connected to a Venturi tube 15 that, when it isfed with compressed air through its entrance P, produces a suitablevacuum in the same chamber, capable of lifting the head/piston assembly1+5.

During the measuring phase the compressed air to the Venturi tube isclosed by an electrovalve (not shown) and it acts as an open passageconnecting the upper chamber 16 to ambient pressure. In this conditionthe head assembly is urged against the sheet by its own weight only,which comprises the weight of the head 1 and the weight of theassociated piston 5.

With reference to FIG. 4 the chamber 14 of the lower head duringmeasuring phase is kept at a given pressure by means of a constant airinput A and a variable discharge regulated gravimetrically. The internalpressure is controlled at a level such that a net force equal to thetotal weight of the upper head is applied also to the lower head. Duringthe non-measuring phase the air supply is closed in order to let thelower head fall away from the sheet by means of gravity.

With reference to FIG. 2 and FIG. 3 the internal hollow of each sensinghead 1 and 2 of circular shape, is maintained under pressure by a commonpressurized air supply (not shown). Multiple air jets are emittedagainst the sheet surfaces from the base of each sensing head throughcalibrated holes 9 symmetrically placed. Each hole is at the center of acircular recessed area a surrounded by an annular barrier 10 protrudingtoward the sheet with its base 11 contiguous to the sheet surface shapedas a flat annular band extending outwardly in the radial direction sothat its area is substantially larger than the area of the internalrecessed land a, typically three times larger or more, so that the forcedue to the pressure field applied to the annular band is the predominantpart of the total force exerted by the air cushion, thus the effectiveair cushion region is delimited by the outer periphery 12 of thebarrier. The annular band 11 and the contiguous sheet surface portionform an extended restricted passage for the air escaping outwardly fromthe inner recessed land, causing a relevant pressure drop due to theassociated laminar friction and a competing significant compressioncaused by the air velocity decrease due to the radial structure of theair stream. With said special geometry the pressure pattern of the wholeair cushion region is mainly determined by the two mentioned phenomena.

FIG. 2a gives an approximate indication of the behavior of the pressureof said air cushion. The pressure of the air escaping under the annularband must decrease so that a positive pressure field is establishedproviding the necessary repulsive force, which means that the effect dueto laminar friction must have a larger magnitude than the one due to airvelocity decrease.

Being that the magnitude of the two above mentioned phenomena arerelated to the air flow rate and consequently to the feed air pressure,but that the effect due to laminar friction is only proportional to theflow rate, whereas the effect related to the decrease in air velocity isproportional to the square of said flow rate, with a proper selection ofthe geometrical parameters, i.e., of the internal and external radius ofthe annular barrier 10, and of the pressure of the common air supply, itis possible to obtain a working point such that the derivative terms ofthe dynamic effects associated to said two phenomena compensate eachother. In this condition even variations of ±10% of supply air pressurearound its nominal value will cause only negligible variations of theair cushion repulsive force. The air cushions of each sensing head aresymmetrically placed around its center at a suitable distance from it toleave enough space to the centrally located inductive sensor, which mustbe out of their pressure fields.

The downward, sheet-approaching force applied to the upper sensing headduring measuring mode is provided by its own weight which balances therepulsive force of the frontal air cushions. In order to obtain the samedegree of invariability also for the upward, sheet approaching forceapplied to the lower sensing head the following arrangement has beenadopted.

With reference to FIG. 4 the lower chamber is fed by a constant air flowthrough passage A, the internal air discharges to the outside throughthe slit S. The aerodynamic resistance of the latter is dependent on thevertical extension of its opening h which in turn depends on thevertical position of the sliding cylindrical shutter C. The verticalposition of this last element is determined by the balance of theexternal forces applied to it which include its own weight and the forceexerted by the pressure of air applied to its bottom base. Therefore theinternal pressure of the lower chamber will be always proportional tothe weight of said shutter, independent from any other physicalparameter as input air flow, air viscosity . . . By choosing the weightof the shutter corresponding to a pressure of the lower chamber suchthat its force applied to the piston balances its total weight plus theweight of the upper head piston, the net upward, sheet approaching forceapplied to the lower sensing head will be equal to the downward,sheet-approaching force of the upper sensing head, and unvariable.

I claim:
 1. A measuring device for scanning from edge to edge andmeasuring the thickness of an industrial continuous sheet, comprising;upper and lower disk-shaped hollow sensing heads facing each other onthe opposite sides of the sheet, said sensing heads being pressurized bycompressed air from an external source, and being separated from thecontinuous sheet surfaces by air layers produced by multiple aircushions, wherein said sensing heads are mounted on pistons which slidealong a path perpendicular to the sheet plane and are guided by the boreof a pipe connected to a sealed rear chamber housing the rear portion ofeach associated piston; each sensing head being equipped with a magneticsensor centrally located which generates a signal proportional to thedistance between the sensing heads from which a measurement of thicknessof the sheet can be derived; each sensing head further comprising anumber of air cushions placed on its base facing the sheet andsymmetrically located at a suitable distance around the center to leaveenough space for the central magnetic element to be out of the aircushion pressure fields; each air cushion being formed of an air jetoutcoming from a calibrated hole located at the center of a circularrecessed land surrounded by a flange-shaped annular barrier protrudingtoward the sheet, the annular barrier having a base next to the sheetsurface shaped as a flat annular band extending outwardly in the radialdirection so that its area is substantially larger than the area of theinner recessed land; the annular band and the contiguous sheet surfaceforming an extended restricted passage for the air escaping outwardlycausing a relevant pressure drop due to laminar friction and asubstantial concurrent compression caused by air velocity decrease dueto the radial structure of the air stream so that the air cushionpressure pattern is mainly determined by said two phenomena, and theeffective air cushion region being delimited by the outer periphery ofthe annular band, said two phenomena producing opposite effects tocompensate variations of fed air pressure around its nominal value,whereby no variation of repulsive force exerted by the air cushions atconstant air cushion is produced; said repulsive force acting on theupper sensing head being counteracted by the weight of the upper sensinghead-piston assembly, with the associated rear chamber being open to theambient pressure, whereas said repulsive force acting on the lowersensing head being counteracted by the action of the lower sensing headpiston caused by the pressurized air in the associated rear chambermaintained at constant pressure by a pressure regulator such that a netupward, sheet-approaching force applied to the lower sensing head equalsthe downward, sheet-approaching force applied to the upper sensing head.2. The measuring device according to claim 1, wherein the pressureregulator of a lower rear chamber is gravimetrically controlled andintegrated in the chamber by means comprising a vertical pipe having alower portion sunk in the rear chamber with a bottom opening incommunication with the internal air of said chamber, an upper portionextending upwardly above the rear chamber with its upper end open to theambient pressure, having a thin vertical slot communicating with theambient air on its lateral wall near its bottom; a cylindrical shutterhoused within said pipe, the vertical position of the shutterdetermining the vertical extension of the passage for the escaping airthrough the slot and consequently the effective resistance encounteredby the air flow from the chamber to the outside so that the pressureestablished in the rear chamber, fed with a constant flow of air from anexternal source, is dependent on the vertical position of said shutter,being in equilibrium at a condition in which its weight is balanced bythe force exerted to its bottom base by the pressure in the chamber,while the weight determines an internal pressure in the lower rearchamber that produces a force to the piston equal to the sum of theweights of the upper and lower sensing head-piston assemblies.
 3. Themeasuring device according to claim 1 capable of retracting the sensingheads into a non-measuring condition by means comprising the upper rearchamber being connected to the output of a Venturi tube, which, innon-measuring condition is fed with pressure air producing a vacuum insaid rear chamber capable of lifting the upper head; the input fed airto said tube being closed by means of an electrovalve so as the Venturitube acts as an open passage connecting the upper chamber to ambientpressure letting the upper sensing head approach the sheet by means ofits own weight in measuring condition, the retraction of the lower headbeing caused by the closing of the fed air to the pressure regulatorproducing positive pressure in the lower rear chamber by means of anelectrovalve so that no positive pressure is produced in said chamberwhich allows the lower head to retract from the sheet by means ofgravity in the non-measuring condition.